The present invention relates generally to airfoils for internal combustion engines, such as gas turbines, and more particularly to gas-turbine castings using ceramic cores with a smooth, craze-free surface for improving the surfaces within the cooling passages of the metal casting.
According to the Department of Energy, natural gas turbines are expected to make up more than 80% of the power-generating capacity to be added in the United States over the next 10 to 15 years. Of the more than 200 new power plant projects announced recently in the United States, 96% plan to use natural gas and most will employ gas turbines.
A turbine is a rotary engine that uses a continuous stream of fluid to turn a shaft that drives machinery, such as the rotor of an electric generator. A gas turbine generally consists of a compressor, combustor, and turbine. Part of the turbine drives the compressor, which draws in large quantities of air, compresses it, and feeds the high-pressure air into the combustor. There the air is mixed with a fuel, such as natural gas, kerosene, or gas derived from coal. The mixture is burned, providing high-pressure gases to drive the turbine. Conventional land-based gas turbines used for power generation are 33 to 40% efficient when used in “simple cycle” mode-that is, without a recuperator or steam generator. Because of the high temperatures in the turbine, most of the airfoils include internal cooling channels, formed by casting.
In a typical airfoil metal casting process, ceramic cores, generally silica-based, are used to produce cooling passages in the airfoil metal casting. Silica cores used for gas turbine applications have complex geometries with large variations in cross-section thickness. Current cores are prepared by using an alcohol-based, gel-casting process.
The current process requires that the cores are dried prior to die injection. In order to achieve acceptable projection rates, a proprietary drying technique is employed. Due to the differential drying shrinkage between the surface and the bulk of the core, the surfaces tend to crack, yielding a crazed surface finish. After subsequent firing, the metal is cast around the ceramic core, and then the ceramic core is leached out using a strong acid. However, the crazed surface finish has been transferred to the metal. By casting with cores that have a smooth surface finish, the quality of the cooling passages, in addition to the mechanical integrity, should be vastly improved.
Conventional silica low-pressure core fabrication provides adequate die release and elimination of surface defects. However, since current silica core technology is based on alcohol containing slurries, for environmental and EHS considerations it is desirable to have a water-based system.
Another property of a good casting system is the type of porosity created when the part is formed since more closed porosity limits the binder removal rate. For example, slip casting and gel casting both produce open continuous porosity prior to binder removal. However, slip cast parts may take up to a week to dry sufficiently to be handled because the parts dry by capillary action. In addition, slip cast parts are difficult to produce having the tolerances required for gas turbine applications.
Water-based, gel casting is a recently developed technique pioneered by Janney et al. of Oak Ridge National Laboratories. U.S. Pat. Nos. 4,894,194 (issued Jan. 16, 1990); 5,028,362 (issued Jul. 2, 1991); 6,066,279 (issued May 23, 2000); 6,228,229 (issued May 8, 2001); 6,365,082 (issued Apr. 2, 2002) and Patent Application Publication U.S. 2001/0003576 (published Jun. 14, 2001) disclose various aspects to producing water-based, gels and castings. The disclosures of which are hereby incorporated by reference in their entirety.
Water-based, gel casting offers the best potential for mechanical properties and viscosity. In addition, water-based, gel casting offers the advantages of material dispersion, die release and dimensional reproducibility. Finally, ceramic cores prepared using water-based, gel casting can be dried using more conventional drying techniques such as controlled-humidity drying or freeze-drying.
Thus, there remains a need for a method to produce gas-turbine alloy castings having craze-free cooling passages which sets up much more quickly than slip casting while, at the same time, avoids the binder burn-off cracking problem associated with current injection molded, alcohol-based binder systems.